Updated perl-Authen-Captcha package uses randomly generated filenames

Description

An issue in previous versions of perl-Authen-Captcha is that the generated
public string (file name of the picture) for the captcha is merely a
checksum of the secret string. It is trivial to break such short strings
even using google instead of a rainbow table.
This new version of perl-Authen-Captcha fixes the problem by producing a
random filename for the captcha.